1. Field of the Invention
The present invention relates to magnetic resonance imaging systems and, more specifically, to an automatic tuning circuit for a quadrature antenna system of a magnetic resonance imaging system.
2. Description of the Related Art
Magnetic resonance imaging ("MRI"), also known as nuclear magnetic resonance ("NMR") imaging, has become a valuable tool as a safe, non-invasive means for obtaining information in the form of images of an object under examination. For example, MRI can be used as a medical diagnostic tool by providing images of the whole or selected portions of the human body without the use of X-ray photography.
MRI systems take advantage of the magnetic properties of spinning nuclei of chemical species found in the observed object in the following manner. Each of the nuclei has an internal spin axis and a magnetic pole aligned with the spin axis. The magnetic pole is a vector quantity representing the magnitude and direction of the magnetic field of the nucleus. Application of an external static magnetic field B.sub.o causes the magnetic poles to align themselves along the external magnetic field lines.
The MRI system disturbs this alignment by transmitting an electromagnetic signal to the object. The magnetic field B.sub.1 of this transmitted electromagnetic signal is circularly polarized and is perpendicular to the static magnetic field B.sub.o. This signal causes the nuclei to precess about the external static magnetic field lines. The frequency of this precession typically is in the radio frequency ("RF") range. More specifically, the precession frequency generally lies within a relatively narrow bandwidth of about 1 to 100 kHz at a center frequency of between 1 and 100 MHz.
As the nuclei precess, they radiate an electromagnetic signal having a circularly polarized rotating magnetic field. The frequency of this rotating magnetic field is generally equal to the precession frequency of the nuclei. The radiated signal is received by the MRI system to produce an image of the object under examination.
The circularly polarized magnetic fields of the transmitted and received RF signals described above rotate in a plane perpendicular to the static magnetic field B.sub.o. For convenience, a rectilinear coordinate system is used here to describe the orientation of these magnetic fields. The static magnetic field B.sub.o is assumed to be in the direction of the Z axis. Therefore, the rotation of the circularly polarized magnetic fields is in the X-Y plane.
Quadrature coil antennas have been used in MRI systems to transmit and receive the RF signals described above. An example of such a quadrature coil antenna is shown in "Quadrature Detection Coils--A Further .sqroot.2 Improvement in Sensitivity," C. N. Chen, D. I. Hoult, and V. J. Sank, J. Magn. Reson., Vol. 54, 324-327 (1983). This antenna includes a cylindrical antenna structure having four saddle coils arranged into a first coil system or pair (two coils) and a second coil system or pair (two coils). The coils of the first coil pair are opposite each other. The coils of the second coil pair are also opposite each other and are oriented 90.degree. relative to the first coil pair.
As the magnetic field produced by the precessing nuclei rotates in the antenna structure, a voltage signal is impressed on the first coil pair corresponding to the component of the rotating field along the X axis while a voltage signal is impressed on the second coil pair corresponding to the component of the rotating field along the Y axis. The signal on the first coil pair has the same frequency as the signal on the second coil pair, but these signals are 90.degree. out of phase, i.e., there is a 90.degree. phase angle between the signals.
In a representative MRI system using a quadrature coil antenna, each of the coil pairs is coupled to a respective antenna coupling circuit. The first coil pair and its coupling circuit comprise a first channel while the second coil and its coupling circuit comprise a second channel. Each of the coil pairs and its coupling circuit comprise a parallel resonant circuit, the resonant frequency of which is adjustable or tunable using a tuning voltage applied at a tuning voltage terminal. Each of the coupling circuits is coupled to a single receiver circuit. The receiver circuit receives the respective signals of the first and second channels, adjusts the 90.degree. phase difference or phase angle between the signals to put them in phase, and combines these signals to produce a single output signal. Noise in the respective channels is assumed to be uncorrelated. Therefore, combination of the signals results in an improved signal-to-noise ratio ("SNR") for the MRI system.
Proper operation of the MRI system as described above requires that the signals of the first and second channels are coherent and in phase when they are combined. This requires the resonant frequency of the first channel to be substantially equal to the resonant frequency of the second channel. A difference in these resonant frequencies or a different phase angle prevents proper superposition of the signals.
The resonant frequencies of the first and second channels in a practical MRI system may vary for a number of reasons. For example, while ideally the design of the first and second coil pairs and their respective coupling circuits is identical, slight variations will inevitably exist, and performance will be affected accordingly. The tuning signal applied to the respective coupling circuits to adjust their resonant frequencies, which ideally is the same for both coupling circuits, may also differ in practice.
In addition, the phase difference of the signals of the first and second channels as they are combined may be non-zero for a number of reasons. For example, the physical alignment of the first coil pair may not be exactly 90.degree. with respect to the second coil pair. Furthermore, the phase delay of one channel may be greater than the delay of the other channel.
Accordingly, it is an intent of the invention to provide an automatic tuning circuit for a quadrature antenna system of an MRI system which maintains a substantial equality of the resonant frequencies of the respective channels of the quadrature antenna system.
It is also an intent of the invention to provide an automatic tuning circuit for a quadrature antenna system of an MRI system which maintains a desired phase relationship of one channel of the quadrature antenna system relative to the other channel.
It is further an intent of the present invention to provide an automatic tuning circuit for a quadrature antenna system of an MRI system which achieves the intentions stated above during operation of the MRI system.
Additional intentions and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description or may be learned by practice of the invention. The intentions and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.